Prostate cancer is a significant health problem among American men. Curative treatments for advanced disease are currently not available and novel treatment approaches are needed. Recombinant forms of TRAIL (tumor necrosis factor related apoptosis inducing factor) have been considered as novel anti-tumor agents but prostate cancer cells are relative resistant to soluble TRAIL. Resistance is overcome by sub-toxic doses of doxorubicin which downregulate the anti-apoptotic protein c-FLIP or by an adenovirus expressing membrane TRAIL. Neither approach is selective for malignant cells and will require restriction of either doxorubicin or TRAIL to a specific target population. It is our hypothesis that combination of doxorubicin with tissue-specific expression of TRAIL from the prostate-specific promotor probasin will be an effective therapeutic approach against prostate cancer. To support this hypothesis and identify underlying mechanisms of TRAIL resistance in prostate cells, the following specific aims will be investigated: (1) To test the hypothesis that the mechanism of doxorubicin-mediated downregulation of c-FLIP involves cell cycle dependent ubiquitination. Cells that are sorted into G0/G1, S and G2/M populations will be analyzed for levels of c-FLIP, ubiquitination, and TRAIL susceptibility. (2) To test the hypothesis that the TRAIL expressing adenovirus (AdTRAIL) overcomes resistance by permitting intracellular interaction of ligand and agonistic TRAIL receptors resulting in formation of an intracellular death inducing signaling complex. The role of c-FLIP in providing resistance to AdTRAIL will be investigated in PC3 cells stably overexpressing long and short isoforms of the protein. Intracellular location of TRAIL receptors and interaction with ligand, caspases and c-FLIP will be analyzed by confocal microscopy. Protein interactions are also determined by immunoprecipitation/immunoblotting. (3) To test the hypothesis that combination of chemotherapy and prostate-specific gene therapy is an effective approach against prostate cancer. The effectiveness of specificity of an adenovirus-expressing TRAIL from the prostate-specific promotor ARR2PB will be tested in cells with a functional androgen receptor and non-prostate cells. The apoptotic potential of virus alone or in combination with doxorubicin will be determined in vitro. The toxicity and effectiveness of AdARR2PBTRAIL alone or in combination with doxorubicin will also be tested in subcutaneous xenografts in athymic mice.